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Australia: The Land Where Time Began |
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Cambrian Explosion The chances of any fossils from more than 500 Ma
being preserved is exceedingly low, made worse most by the organisms
from the Early Cambrian being mostly soft-bodied. The conditions on the
sea floor also vary over space and time so that the conditions conducive
to the preservation of any fossils, but especially soft-bodied fossils,
also vary over time and from place to place. The presence or absence of
other organisms also affects the changes of an animal being fossilised.
An example is the presence of algal mats that can help protect the
remains of dead animals which gives them time to undergo the processes
of fossilisation, remains that would otherwise be damaged or destroyed
by scavengers and traces could be removed by the work of bioturbators.
The chances of preservation of fossils is also affected by the chemical
history of the sediments entombing the fossil remain. The sorts of
biomineralised skeletons forming the bulk of the invertebrate record of
the Phanerozoic, which represent a range of “normal” modes of
preservation, are contained at many Cambrian localities. Skeletal
fossils from the
Neoproterozoic or Cambrian, as with most cases
throughout the Phanerozoic, have often had their minerals replaced by
different materials. Distinctive preservation styles characterise
unusual body fossil assemblages that the authors1 describe as
particularly striking enhance the record of the Cambrian Explosion.
Original organic carbon films represent many of the fossils from this
time (Butterfield, 1990, 1995). This type of preservation is named for
the Burgess Shale Fauna of British Columbia, Canada and is also found in
the Chengjiang Fauna from Yunnan, China, that is earlier (Butterfield,
1990, 1995; Gaines, Briggs and Zhao, 2008). In this style of
preservation the usual processes of bacterial decay are suppressed, the
authors1 suggesting probably by anoxic conditions. A large
proportion of fossils reviewed in the chapter are from these and other,
similar faunas. Another especially important type of preservation found
in these deposits from the Early Cambrian is of very small shells found
in phosphate deposits, the shells being either originally phosphatic or
their constituents have been replaced by phosphate minerals during
fossilisation. A different fraction of a biota is preserved by different
accumulation and fossilisation histories of organisms. A range of these
histories have been preserved in the known deposits from the Early
Cambrian. The richness of these faunas gives some indication of how much
is missing from times where only “normal” preservation modes are
represented.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |